In a conventional valve lift controller, several types of actuators are used for linearly driving an axis of a changing mechanism which controls a lift amount of a valve based on a position of the axis. For example, an actuator is described in US 2004-0083997A1 (JP 2004-150332A) which converts, by means of a reduction mechanism and a cam mechanism, a rotational driving force into a linear driving force and apply the linear driving force to the axis of the changing mechanism.
The conventional actuator, however, has to use the reduction mechanism in combination with the cam mechanism to make the linear driving force strong. It is therefore difficult to design the actuator to be small. Thus, positions where the actuator can be located may be limited.
The inventors of the present invention have studied a structure of a feed screw mechanism which converts a rotational movement of a rotation axis to a linear movement of a screwed axis. The feed screw mechanism can generate the strong linear driving force by means of a simple structure in which the rotation axis and the screwed axis are coaxially connected directly or indirectly. An actuator with the feed screw mechanism therefore can be designed to be smaller than the actuator with the reduction mechanism and the cam mechanism.
The inventers, however, found a problem in decreasing the size of the actuator in the case that the feed screw mechanism and the motor unit are installed in the same housing. When lubricating oil is supplied into the housing to lubricate a friction making portion of the feed screw mechanism, the motor unit in the housing receives the lubricating oil. Especially, if the motor unit is designed to drive a spindle by exiting a coil, the lubricating oil causes defect in the motor unit such as disconnection. It is important for improvement of endurance of the actuator to avoid the defect caused by the lubricating oil.